I might as well admit it. When Madhur Jaffrey, queenly in an outsized pair of sunglasses, arrives at the St John Hotel in Leicester Street my heart sinks. She is tiny! Can she really be a lunch kind of a person? More to the point, can she really be a St John kind of a person? (She chose the venue.) This is my first visit to Fergus Henderson's latest outpost – a hotel where guests will be able to get their hands on a fresh bun at any hour of day or night – and I am hoping to enjoy myself, by which I mean, of course, that I am hoping to eat as much as possible within the confines of politeness and the fact that, later on, I have important duties back at my desk. Frankly, my day is going to be ruined if Jaffrey, so slender and so immaculate, orders a green salad and a glass of fizzy water.

Fortunately, it turns out that the woman who taught the British to make curry is an eater. Quite a big eater. While I order weedy lunchtime food – barley, carrots and curd, followed by brill with fennel and salt lemon – Jaffrey goes for the full St John deal: lamb sweetbreads with butter beans and wild leeks, and then tripe and onions, with mashed potato and watercress on the side. "Yummy," she says, several times, while we are waiting for all this to arrive. "I love this kind of food." When the waiter asks us if we want wine, and I feebly shake my head (on the grounds of the aforementioned duties), she says: "I will if you will," and places the list silently in my hands. In honour of the weather, we choose rosé. What is she doing this afternoon? "I'm cooking," she says. "A great feast for a photo shoot." She smiles. "How on earth am I going to get through it?"

When Jaffrey first came to London from India – she wanted to act, and had won a place at Rada, where a young Diana Rigg was among her contemporaries – it was the late 1950s; rationing had only recently ended. Did British food come as a shock? "The city was very drab, but I do remember the wonderful fishmongers," she says. "One on almost every corner. All gone now, of course. And the fish and chip shops, where you could eat seriously good, seriously fresh fish. Otherwise, yes, there wasn't anything much to eat. But I was young, and I had embarked on what I thought of as my first great adventure. So I learned to drink, and to smoke, and..." She shrugs, as if to suggest a diet of nothing more than fresh air. This, of course, is only half the story. Quietly, she also began cooking. At her digs, where she was blessed with a landlady who didn't give a fig what she did in her kitchen, she practised the recipes her mother posted from Delhi. What, for her, was the taste of home? "Rice and dahl," she says. "So simple, but so delicious."

When she visits London now, she can hardly get over the choice available, though she tends to eat in only one place: Barrafina, the Soho tapas bar. "They don't know who I am. I queue up with everyone else. I don't mind at all. It's worth the wait. They are the best tapas I've had – and I've been to Spain." What about curry? "People are always asking me where to go for a curry in London. But why would I know? I never go out for a curry." At home in New York – she has a flat in Manhattan and a farmhouse upstate – she never eats out at all. "My husband [Sanford Allen, a musician] and I take turns to cook. We work on opposite sides of the hall, and throughout the day we'll call out to one another: what shall we eat tonight?" Allen is from the south, and likes to cook the food of his childhood: biscuits, collard greens, black-eyed peas. Jaffrey will cook Thai, or Japanese, or Korean... "anything, really". It is, she admits, an enviably idyllic life. Upstate, she has created a little corner of India, cultivating hibiscus, rhododendrons, jasmine and azaleas. The couple also grow their own fruit and vegetables. Their kale has been a great success, but when they dug up their salsify – "so disappointing" – it was as thin and as knobbly as a piece of ginger left for too long at the back of the fridge (not that this happens in the Jaffrey household).

She and Sanford met more than four decades ago, in New York, to which Jaffrey had moved with her first husband, the actor Saeed Jaffrey (they divorced in 1965). "I wasn't picking up much work as an actor, so I had a job as a tour guide at the Lincoln Centre; Sanford was a violinist with the Philharmonic, which is based there. I remember our first date vividly. He took me to a bistro nearby, and we had trout amandine. It's a dish that has almost disappeared. I don't know why because it's absolutely delicious. It's the same with crepes. Why doesn't anyone serve crepes any more?"

Her career in cooking – "completely accidental" – was also the result of needing to plug the gaps between acting jobs. She was doing bits and pieces of food writing – it started with an article in the New York Times to promote her second film, Shakespeare Wallah, directed by James Ivory – and running cookery classes at her apartment. In 1973, this led to her first book, the award-winning Invitation to Indian Cooking. A decade later, word reached her that the BBC was looking for someone who could do Indian cookery on television. Jaffrey recorded the bustling sound of one of her classes, sent it to London, and was promptly commissioned to make a series – at which point, British housewives began buying her books in their thousands. She has now published 15; most have her name in their title.

While she eats her tripe – she does so extremely slowly; it is rather rich, but she is also reluctant to leave any – we talk recipes. This is when she really comes alive. Her most recent book, Curry Easy, is an attempt to persuade people that Indian food can be cooked quickly, with the minimum of fuss. It has been a hit. But she knew she was on to something when she went away, and Sanford used one of its recipes. "He'd never made curry before on the grounds that I always did it, and also he insisted that I never label my spices – which is not true." The key to good, fast Indian food is, she thinks, to spice your meat the day before, and leave it to marinate overnight – though when Jaffrey has large numbers to entertain, she likes to make a kind of Indian shepherds pie: layers of aubergine and tomato, topped with keema (spiced minced meat), topped with spiced potatoes. "Try it," she says. "It's so good."

The tripe has finally disappeared. Is she full? "Yes." She pats her stomach. "But still, I think we should share a pudding." We order rhubarb trifle, with two spoons. She had told me she does not have a particularly sweet tooth, but her spoon dips in as swiftly as mine.

Her family was always supportive of her desire to act, but her cookery career amazed her mother. "My father [he owned a ghee factory] was from a grander family than my mother, and when they married, she was told she would not be cooking; there would be other people to do that. So I never really saw her cook at home, and I didn't do any cooking until I came to London. So when I became this so-called authority on Indian food, well, my mother thought it was rather funny." When she meets people who cook with books of hers that originally belonged to their mothers, or even grandmothers, it is thrilling, but she still feels – just a little – like an imposter. She is drinking her espresso now (decaff, to my amazement). What will she do next? She is 77(not that you would know it). "A vegetarian book, I think," she says. "I want to travel the length and breadth of India, and find all the most delicious things." It's 3pm. She is replete. She is even, she admits, a little sleepy. But still, her cat-like eyes widen delightfully at the thought.